Photoreceptors are responsible for transforming light energy impinging on the retina into neural signals, and therefore information contained in the stimulus but not transmitted by the photoreceptors is lost to the organism. For this reason, knowledge of the response characteristics of the photoreceptors not only provides information about the receptors themselves, but is also a necessary prerequisite to an understanding of more central processing in the visual system. Microelectrode recording techniques within the retina are capable of factoring out the receptor response, in particular the late receptor potential, from the other retinal units, and these methods applied to cynomolgus macaque monkeys, which have human-like visual systems, provide an approach to the study of photoreceptor signal processing in man. Accordingly, the foveal local ERG will be used as an indicator of cone activity to investigate the role photoreceptors in various perceptual phenomena. Recent advances in microelectrode technology will be applied in an attempt to achieve intracellular recording from primate cones. BIBLIOGRAPHIC REFERENCES: Baron, W.S. (1977), The foveal local ERG response to transient and steady state flickering stimuli, Proceedings 14th ISCERG Symposium, in press. van Norren, D. and Baron, W.S. (1977), Increment spectral sensitivities of the primate late receptor potential and b wave. Vision Research, in press.